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C H I L D H O O D D E V E LO P M E N TA L D I S O R D E R S
What developmental disorders can tell us
about the nature and origins of language
Gary Marcus & Hugh Rabagliati
Few areas in the cognitive sciences evoke more controversy than
language evolution, due in part to the difficulty in gathering
relevant empirical data. The study of developmental disorders
is well placed to provide important new clues, but has been
hampered by a lack of consensus on the aims and interpretation
of the research project. We suggest that the application of the
Darwinian principle of ‘descent with modification’ can help
to reconcile much apparently inconsistent data. We close
by illustrating how systematic analyses within and between
disorders, suitably informed by evolutionary theory—and ideally
facilitated by the creation of an open-access database—could
provide new insights into language evolution.
Language is arguably the defining characteristic of the human species,
yet the biological basis of our ability to speak, listen and comprehend
remains largely mysterious; about its evolution, we know even less. As in
other areas of cognition, comparative animal research can yield valuable
insights1–4, but the study of the nature and origin of human language
poses special challenges. No system of animal communication approaches
human language in its complexity, and many of the powerful techniques
that have been used in animal studies (for example, gene knockout studies
and the ‘deprivation studies’ used to study how isolated songbirds develop
songs) cannot ethically be applied to human beings.
Human developmental disorders could offer special insight into the
genetic, neural and behavioral basis of language because they provide
a way to study naturalistically what cannot be controlled in the lab. For
example, studies of developmental disorders have been particularly
prominent in a central issue in cognitive neuroscience: the relation
between the biological (and psychological) basis of language and the
biological (and psychological) basis of other cognitive or neural systems. One classic view holds that language is a “modular system” that
should be studied largely on its own terms, and another view is that
language is simply a particular byproduct or instantiation of powerful “domain-general” cognitive systems. Advocates of both views have
pointed to studies of developmental disorders. One set of studies,
invoked by critics of modularity, has shown that impairments in language often co-occur with impairments in other spheres of cognition,
such as motor control5 and general intelligence. Variability that can be
Gary Marcus and Hugh Rabagliati are at theDepartment of Psychology,
New York University, 6 Washington Place, New York, New York 10003, USA.
e-mail: [email protected]
Published online 26 September 2006; doi: 10.1038/nn1766
1226
attributed to genetics in one domain (say language) also typically correlates strongly with genetically attributable variability in other domains6.
Such correlations may indicate that language is mediated largely by
“generalist genes” and therefore, “genetic input into brain structure
and function is general, not modular”6.
Advocates of modularity have focused on what we will call dissociability. One prominent case study7, for instance, focused on a single
10-year-old child, AZ, with a particular grammatically focused form
of specific language impairment. AZ showed a significant deficit in
language comprehension and production, while otherwise showing
normal cognitive functioning. On tests of auditory processing, analogical and logical reasoning as well as nonverbal I.Q., AZ performed as
well as age-matched controls. In contrast, he frequently omitted grammatical inflections (for example, the plural –s) and proved unable to
use or properly understand subordinate clauses or complex sentence
constructions. Likewise, in sentences such as Grandpa says Granny is
tickling him, AZ could use context to accurately infer the referent of
him (i.e., Grandpa), but where context alone was inadequate, AZ performed at chance. In the sentence Mowgli says Baloo is tickling him,
a normal native speaker recognizes that the pronoun him can only
refer to Mowgli; conversely in Mowgli says Baloo is tickling himself, a
normal speaker recognizes that himself must refer to Baloo. Despite
normal intelligence, AZ was never able to make such distinctions. The
authors concluded that “The case of AZ provides evidence supporting
the existence of a genetically determined, specialized mechanism that
is necessary for the normal development of human language.”
We very much admire both lines of work—the care taken by modularity’s critics in investigating the overlap between language and other
aspects of cognition, and the detailed case studies of how specific aspects
of language can be dissociated within well-defined subgroups, as undertaken by modularity’s advocates. (There is also interesting work on the
opposite sorts of cases such as Williams syndrome, in which afflicted
members have marked deficits in domains such as spatial cognition,
but comparatively spared language8,9.) At the same time, the mutual
inconsistency of the two conclusions is striking.
One sees a similar lack of consensus in discussion of the significance
of the gene FOXP2, initially noted in studies of a severe inherited
speech and language disorder10–12 found in the British KE family13.
Early debate (before the gene was identified) focused on the specificity
of the disorder, which apparently afflicts both language and orofacial
motor control13. Recent debate has focused on the relation between
FOXP2 and recursion, the ability to embed and conjoin units of a language (such as phrases or words), which underlies much of modern
linguistic theory. Advocates of a link between the two have argued that
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FOXP2 may “confer enhanced human reiterative ability [recursion] in
domains as different as syntax and dancing14.” In contrast, one recent
high-profile paper states that it is highly unlikely that FOXP2 has anything to do with recursion15. A third perspective16, meanwhile, uses the
wide-ranging effects of FOXP2 expression13 to criticize the suggestion4
that recursion might be the only mechanism unique to both humans
and to language, suggesting that the FOXP2 facts “refute the hypothesis
that the only evolutionary change for language in the human lineage
was one that grafted syntactic recursion onto unchanged primate inputoutput abilities.” Among these three reports, one sees FOXP2 as essential
for recursion and by extension language evolution, one sees that FOXP2
as not important for what is unique about human language, and a third
provides evidence that recursion is not the only unique contribution to
language. As FOXP2 is expressed in many species apart from humans17,
and as the gene is expressed in the lungs as well as as the brain,18 the
situation is clearly complex19.
In our view, although developmental disorders hold great promise
for informing these debates, no firm consensus has yet been reached.
We raise these inconsistencies not to cast doubt on the possibility of
useful data emerging from developmental disorders, but to suggest that
a change in perspective may be necessary.
A more plausible perspective?
We suggest a return to first principles—specifically the logic of evolution,
or what Charles Darwin called “descent with modification”. As Darwin
wrote, “the existing forms of life are the descendants by true generation
of pre-existing form.” That is, the systems that currently exist have a twopart evolutionary history: something borrowed, something modified.
Although the “survival of the fittest” strand of Darwin’s thought
has, in the form of adaptationism, often been prominent in evolutionary approaches to psychology, the importance of descent with
modification has generally received less attention. A focus on adaptation alone appears to be implicit, for example, in a suggestion from
evolutionary psychology that “There is no more reason to expect
any two cognitive mechanisms to be alike than to expect the eye
and the spleen, or the pancreas and the pituitary to be alike.”20 But
this position overlooks the importance of the causal mechanism of
descent with modification. Language evolved only relatively recently,
probably within the last few hundred thousand years, presumably out
of other neural substrates. It would thus seem unlikely that language
and cognition would really be as distant as eye and spleen (which
have been diverging for roughly a thousand times as long).
At the same time, the element of modification implies important
evolutionary change, even where two systems may stem from common
origins; as a consequence, “generalist genes”21 on their own are unlikely
to be adequate22,23. Consider for example the forelimb and hindlimb.
There is enormous overlap in which genes are expressed24. Yet that overlap is less than total, and differences between hindlimb and forelimb
are presumably largely attributable to the small genetic differences. In
a similar way, differences between language and cognition may depend
on small amounts of change, genetic, neural and psychological, relative
to a largely shared common background.
Seen in this light, one might imagine three caricatures (Fig. 1),
depicting how the different perspectives on language evolution would
predict the relations between the genes, neural substrates and cognitive
substrates underlying language and the full range of other cognitive
systems. In our view, the logic of descent with modification clearly
points to the relation seen in Figure 1c.
Provisional acceptance of this perspective has important
implications, both theoretical and empirical, for how the nature and
origin of language should be studied.
NATURE NEUROSCIENCE VOLUME 9 | NUMBER 10 | OCTOBER 2006
Figure 1 The relation between language and cognition under three
evolutionary scenarios. (a) The genes, neural substrates or cognitive substrates
underlying language are largely separate from the genes, neural substrates
or cognitive substrates underlying other neural systems. (b) A scenario in
which language would depend on a subset of the machinery used for building
cognition in general. (c) A scenario, analogous to the hindlimb/forelimb case,
in which the machinery underlying language would largely but not entirely
overlap with the machinery that underlies other neural/cognitive systems.
On the theoretical side, the logic of descent with modification speaks
against the two extreme views that currently characterize the debate.
On the one hand, descent with modification argues against “sui generis
modularity,” according to which modules are treated as independent neurocognitive entities that owe nothing to one another25; on the other hand,
it suggests that exclusive study of overlapping “generalist” contributions is
likely to miss some of the most important evolutionary contributions. On
this view, language must be understood as the joint product of domaingeneral ancestral inheritance and domain-specific adaptations.
Empirically, it follows that developmental disorders are particularly
well placed to yield insight into the evolution of language by providing insight into both halves of the equation: that which is unique to
language, and that which is not. The logic of descent with modification
predicts that impairments of particular aspects of language will correlate with impairments in their ancestral cognitive structures. As such,
it follows that a great deal can be learned by comparing impaired and
spared language and cognition in individuals both within and between
disorders over the course of development.
As an example of a within-disorder comparison, consider word
learning in children with autism. Several researchers have recently
suggested that to a large extent, learning the meanings of words is
driven by an understanding of the intentions of other speakers26,27.
Accordingly, children with autism, generally considered to have a deficit in understanding other people’s intentions (a “theory of mind”)28,
have difficulty learning the names of objects when cues such as eye gaze
are crucial29,30. In most contexts, however, the capacity to infer other
people’s intentions might be merely helpful, not essential. A number of
researchers have suggested that children might also be able to use logical
reasoning strategies, independent of social apprehension, to infer the
referents of words31,32. Consistent with this idea, autistic children do
not appear to be impaired on word learning tasks that only require them
to use a reasoning strategy known as “mutual exclusivity” (presuming
that each object tends to have only one label)30, suggesting two distinct
evolutionary contributions to word learning, one from general capacities for logical reasoning33, the other from sophisticated mechanisms
evolved for the purpose of social cognition34.
According to this perspective, a series of intriguing correlations
should emerge within the domain of word learning. First, the ability to
learn words when the most critical cues come from the social situation
should be positively correlated with performance on theory-of-mind
tasks; yet this ability should also be comparatively independent of performance on logical reasoning tasks. At the same time, correct use of
strategies like mutual exclusivity should be correlated with performance
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PERSPECTIVE
on (nonlinguistic) logical reasoning tasks even after the ability to use
referential intent as a cue to meaning is factored out. Alternatively,
if word-learning depended only on general reasoning capacities, one
would expect positive correlations between word-learning ability and
general intelligence, with no additional variance attributable to performance on theory of mind tasks. The opposite pattern would be
expected if word learning were solely descended from structures supporting social cognition rather than general intelligence.
Patterns of dissociation and comorbidity between disorders can cast
further light. For example, consider the relation between fragile X syndrome, Down syndrome and Williams syndrome, each of which impairs
cognitive capacities, but in markedly different ways. In fragile X syndrome,
an X chromosome–linked disorder common in males, cognitive abilities
are significantly compromised, but receptive language skills are typically
on par with those of normal children of equivalent mental age, albeit with
a number of pragmatic difficulties35–38. Within Down syndrome, a disorder resulting in physical and learning disabilities linked to chromosome
21, language is delayed relative to mental age39,40. In Williams syndrome,
another genetic disorder resulting in a broad range of cognitive impairments, language is significantly advanced relative to mental age8,9.
In themselves, these three disorders point to a degree of dissociability between language and cognitive skills; closer examination may
reveal considerably more. For example, one component of the linguistic impairment in fragile X syndrome37, perseverative language,
may be attributable not to a problem with, say, grammar, but rather
to a difficulty with (a domain-general process of) inhibitory control,
which suggests that evolution co-opted a domain-general process
(inhibition) for at least one aspect of the language faculty. In other
cases, such as in the relation between impaired spatial cognition and
impaired language, there appears to be no systematic pattern either
within or between disorders8,9,41–44, suggesting (contrary to a common
suggestion) that the ability to represent relations between syntactic
and semantic elements evolved separately from the ability to represent
relations between physical elements in space.
Fine-grained data on how developmental disorders impair (and
leave intact) both cognition and language could thus lead to important
insights into the ways in which language builds on and departs from
particular aspects of domain-general cognition.
Maximizing what we can learn from developmental disorders
The contribution of developmental disorders to our efforts to reconstruct the nature and origins of language could be considerably
enhanced through the creation of a large-scale open-acess database.
Although current data already provide some clues about comorbidity
and dissociability, the extant data are largely proprietary (rather than
publicly available), fragmentary (in that most studies focus either on
comorbidity or on dissociation, but not both) and unstandardized (in
that fine-grained assessments of particular linguistic or cognitive functions typically depend on ad hoc tests designed in individual labs. To
a certain extent, even when asking the same questions (for example,
about the relationship between phonological and syntactic impairments), each lab uses its own measures. Furthermore, the samples from
which these data are drawn are not necessarily representative, as groups
of subjects are often selected based on intrinsic interest (for example,
because particular subjects are twins, or precisely because those subjects
face problems with language that do not immediately appear to be due
to other cognitive limitations). As a consequence, reliable cross-study
estimates of comorbidity and the prevalence and nature of strong dissociations are difficult or impossible to reconstruct.
The logic of descent with modification, however, suggests that
fine-grained data drawn from anonymous individual profiles
1228
of particular children could be critical to elucidating rare but
theoretically important developmental trajectories. A composite
measure of cognitive ability, for example, might not correlate with a
composite measure of language, even though deficits in more specific
cognitive abilities such as inhibition might correlate with a specific
deficit in language such as perseveration.
At present, it is remarkably difficult to answer basic questions such
as “what proportion of children with autism have difficulty with
inflectional morphology?” Certainly no publicly available data allow a
researcher to ask “how does the capacity of autistic children to acquire
inflectional morphology relate to their capacities to learn the meanings
of concrete nouns as opposed to words describing abstract personal
relationships?” There is likewise no way to gather comparable data for
children with Williams syndrome, Down syndrome or fragile X. Despite
the plethora of quantitative data available from individual studies,
answers to specific theoretical questions continue to rely on data that
is often anecdotal, distributed and possibly unsystematic.
A systematic, publicly shared database of developmental disorders,
combined with a capacity to tap (or at least request) pedigree data and
genetic data, could provide a powerful new tool for investigating the
origins of language. It would not supplant traditional approaches such as
comparative psychology, developmental psycholinguistics or neuroimaging, but in combination with them, could yield considerable insight.
Conclusions
The comparative study of language disorders provides researchers with
a natural experiment, a rare chance to examine how variance within the
genome influences cognition. The variation and covariation in abilities
seen between the normal population and those with language disorders,
as well as between those with different disorders, provides a situation
logically equivalent to the results of a knockout study. Much as knockout studies can tell us about common heritage in different abilities,
studies of language disorders could tell us about the diverse heritage
of the many different aspects of language.
Whereas initial studies typically merely described disorders45,46, more
recent approaches have emphasized the importance of causal factors that
change over the course of development47. Our proposal is complementary
to this, and emphasizes how vital it is to study both what is impaired and
what is unimpaired within a disorder, over the course of development.
As an example, evidence that auditory processing deficits cause certain
forms of specific language impairment48 would show that language is
dependent upon a correctly functioning auditory system, but would
not lead to many further insights on language evolution. In contrast, a
detailed comparison between the phenotypes of two particular disorders
where language skills show analogous impairments but cognitive patterns
are very different (for example, Down syndrome and specific language
impairment49) could yield an excellent testing ground for tying nonlinguistic abilities to their possible language counterparts.
Many challenges remain; for example, cognitive capacities may dissociate only at particular points of time47; because of the possibility,
indeed inevitability, of change over development, the growth of language and related cognitive systems must be studied dynamically, rather
than statically. Studies of disorders do not obviate the need for careful
linguistic analysis, for neuroimaging, or for careful analyses of both preand postnatal environmental input22,47. Nevertheless, through careful
cross-disorder comparison, we can begin to discern which symptoms
of language disorders are necessarily causally related, which are correlated by virtue of their shared mechanisms, and which are correlated
accidentally, for example simply by being linked to genes with proximal
loci. In this way, through careful, systematic analyses of co-morbidity
and dissociation, developmental disorders have the potential to provide
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© 2006 Nature Publishing Group http://www.nature.com/natureneuroscience
important insight into the nature and evolution of language, and how
language relates to other aspects of cognition.
What developmental disorders reveal about the nature of language
has been a hotbed of discussion for over two decades, but perhaps
stymied by a commitment to extreme views. The logic of descent with
modification suggests a move away from an all-or-nothing perspective
on modularity that could lead to important new insights into the nature
and evolution of language.
ACKNOWLEDGMENTS
We thank S. Fisher and A. Vouloumanos for helpful comments on the manuscript.
This work was supported by grants from the US National Institutes of Health
(HD37059 to G.M.) and the Human Frontier Science Program (to G.M.).
COMPETING INTERESTS STATEMENT
The authors declare that they have no competing financial interests.
Published online at http://www.nature.com/natureneuroscience
Reprints and permissions information is available online at http://npg.nature.com/
reprintsandpermissions/
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